A high-resistance layer between two electrodes provided on opposite two principal faces must be thick in order to achieve the high withstand voltage in a longitudinal semiconductor device, in which an electric current flows between the two electrodes. A device having the thick high-resistance layer provides high ON-state resistance between the two electrodes, and thus, the loss is unavoidably increased. In short, there is a trade-off relationship between the ON-state resistance (the current-carrying capacity) and the withstand voltage. As is well known, the trade-off relationship applies to a variety of semiconductor devices such as an IGBT, a bipolar transistor and a diode. The problem also applies to a lateral semiconductor device in that a direction in which a drift current flows in the ON state is different from a direction in which a depletion layer spreads due to the reverse bias in the OFF state.
To address the above-mentioned problem, European Patent No. 0,053,854, U.S. Pat. No. 5,216,275, U.S. Pat. No. 5,438,215, and Japanese Patent Provisional Publication No. 9-266311 developed by the inventors of this invention have disclosed a semiconductor device having a drift layer composed of a parallel pn layer, in which n regions and p regions with high impurity density are alternately piled up. In this semiconductor device, the parallel pn layer is depleted in the OFF state to burden the withstand voltage.
In the following description, a semiconductor device having a drift layer composed of a parallel pn layer, which conducts electricity in the ON state and is depleted in the OFF state, will be referred to as a super-junction semiconductor device.
The above-mentioned conventional semiconductor device is in an experimental stage, and it is still impossible to manufacture the devices in large quantities. For example, the impurity density and width of the parallel pn layer need to be uniform, but actually, the impurity density and the width are always uneven in the manufacturing process.
Moreover, no specific numerical value has been specified with respect to an L load avalanche breakdown current, which is an important factor in the manufacture of the devices. The L load avalanche breakdown current is preferably more than a rated current in order to manufacture the devices.